Electrostatic capacitance fluid level sensor

ABSTRACT

In an electrostatic capacitance fluid level sensor including fluid level detection electrode plates mounted on conductive pins mounted on a hermetic terminal, a fluid level detection part mounted on the hermetic terminal has a detection unit including two electrodes, electrically insulating spacers fixing a positional relationship between the electrodes, and at least one connecting terminal configured to connect and fix the detection unit to the conductive pins. The connecting terminal is deformable by a weaker force than a force causing the electrodes to deform. As a result, stress applied to the electrodes of the detection unit is dispersed/relaxed. Since the detection unit is further connected and fixed to the hermetic terminal for a motor compressor, a work for mounting the sensor to the hermetic motor compressor can be rendered easier as well as the handling of the sensor.

TECHNICAL FIELD

The present invention relates to an electrostatic capacitance fluid level sensor detecting an amount of fluid, and more particularly to an electrostatic capacitance fluid level sensor having integrally formed hermetic terminals via which the sensor is mounted on an electrically driven compressor and the like.

BACKGROUND ART

Methods of detecting an amount of fluid have been conventionally known. Electrostatic capacitance sensors have been proposed as means for detecting an amount of electrically nonconductive fluid such as oil. This sensor detects the presence of fluid and an amount of the fluid, using changes in electrostatic capacitance between electrodes by causing electrically insulating fluid to penetrate into a space between the electrodes. For example, the electrodes are disposed in a cylindrical casing which is electrically insulating or one of the electrodes is formed into a cylindrical shape. When fluid flows into and out of the cylindrical member, electrostatic capacitance between the electrodes changes. An amount of fluid in the container is measured from the changing electrostatic capacitance.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent Application Publication No. JP-A-H11-311561

Patent Document 2: Japanese Patent Application Publication No. JP-A-H07-260549

SUMMARY OF THE INVENTION Problem to be overcome by the Invention

The electrostatic capacitance fluid level sensors include a type which can detect continuous changes in an amount of fluid on a real-time basis as shown in the foregoing prior art documents. However, the determination to detect a subtle change in the electrostatic capacitance becomes more difficult as the measurement is precise. Furthermore, since the determination changes depending upon a type of liquid and/or a type or size of the container to which the sensor is mounted, actual gauging is required.

For example, FIGS. 6 to 8 show a proposed example of fluid level sensor having a simplified construction by limiting the use of the sensor only to detection of presence of fluid but not a continuous detection of fluid amount. The fluid level sensor 101 includes a substrate 102 to which two conductive pins 104 are fixed. The pins 104 have distal ends to which metal electrode plates 105 are fixed by welding and configured to face each other substantially in parallel with each other.

While the above-described fluid level sensor has a simple structure, a slight tilt of each electrode plate caused by the welding to the conductive pin changes an interelectrode distance. Accordingly, adjustment of interelectrode distance is necessary in order that the tilt caused during the welding may be removed for the purpose of uniforming the sensitivity. In particular, a slight tilt of the welded part has a large influence on the interelectrode distance as the electrodes are rendered long so that areas of the electrodes are increased for the purpose of obtaining electrostatic capacitance. Furthermore, since the cantilevered electrodes are bared, there is a possibility that the electrodes may be deformed when subjected to a force during assembly. Furthermore, when the strength of the electrodes is increased by thickening the electrodes for prevention of deformation, stress concentrates on welded parts which are relatively weaker. This has a possibility of changing the positional relationship of the electrodes.

Even when electrodes are adjusted to a proper positional relationship in manufacture, an increase in the internal pressure expands the substrate in the case where the sensor is mounted to a motor compressor. This slightly changes an angle between electrically conductive pins, resulting in a large change in the distance between electrode plates mounted on the distal ends of conductive pins. Accordingly, the electrostatic capacitance between the electrodes changes with the result of a problem that the performance of the fluid level sensor is reduced.

Furthermore, since two pieces of electrode are welded on the same straight line connecting between the conductive pins, one welding electrode needs to be inserted between the conductive pins. Furthermore, the firstly fixed electrode gets in the way of the welding of the second electrode. Additionally, the interelectrode spacing and the electrodes become slenderer and thinner as the sensor is small-sized, with the result of a problem that a sufficient pressure applied between the electrodes during welding cannot be obtained.

Accordingly, an electrostatic capacitance fluid level sensor is desired which is simple in the construction and easy to manufacture and handle.

Means for Overcoming the Problem

An electrostatic capacitance fluid level sensor in accordance with the invention includes a fluid level detection part mounted on the hermetic terminal and constituted into a detection unit including two electrodes electrically insulating spacers fixing a positional relationship between the electrodes, and at least one connecting terminal configured to connect and fixing the detection unit to the two conductive pins, the connecting terminal being deformable by a weaker force than a force causing the electrodes to deform. As a result, stress applied to the electrodes of the detection unit is dispersed/relaxed. Since the detection unit is further connected and fixed to the hermetic terminal for a motor compressor, a work for mounting the sensor to the hermetic motor compressor can be rendered easier as well as the handling of the sensor.

The connecting terminal is formed of a thinner metal plate than the electrode thereby to be deformable or the connecting terminal includes a bend diverted to a predetermined extent so that the connecting terminal has a length longer than an actual distance between secured portions. As a result, the connecting terminal is deformable by a weaker force than a force causing the electrodes to deform, whereby stress applied to the detection unit can be widely dispersed to the whole connecting terminal.

Furthermore, two connecting terminals are provided, one of the connecting terminals has a sufficient stiffness to hold the detection unit and the other connecting terminal is flexible enough to be deformable, with the result that the strong fixing to the hermetic glass terminal and stress dispersion can be achieved at the same time more easily.

Effect of the Invention

According to the fluid level sensor in accordance with the invention, the detection unit having a fixed positional relationship between the electrodes is constructed as the fluid level detection part, whereby a work for adjusting positional relationship between the electrodes can be rendered easier. Furthermore, the connecting terminal fixing the detection unit can disperse/relax stress caused by error in the assembly of components during the fixing of the detection unit and almost all of the force caused by deformation of the hermetic terminal. This can avoid deformation or breakage of the detection unit due to stress concentration. Accordingly, the performance of the fluid level sensor can be retained even in the use under severe conditions.

Furthermore, since one of the connecting terminals is constructed to have a sufficient stiffness and the other connecting terminal is constructed to be flexible, the reliable retention of the detection unit and dispersion/relaxation of stress can be achieved at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overhead view of an electrostatic capacitance liquid surface sensor in accordance with the invention;

FIG. 2 is a sectional view of the liquid surface sensor as shown in FIG. 1;

FIG. 3 is a sectional view taken along line A-A in FIG. 2;

FIG. 4 is a partially sectional view of the liquid surface sensor in accordance with another embodiment of the invention;

FIG. 5 is a partially sectional view of the liquid surface sensor in accordance with further another embodiment of the invention;

FIG. 6 is an overhead view of a conventional liquid surface sensor;

FIG. 7 is a sectional view of the liquid surface sensor in FIG. 6; and

FIG. 8 is a front view of the liquid surface sensor in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

An electrostatic capacitance fluid level sensor 1 in accordance with the present invention is constituted by a metal hermetic terminal 2 for use in mounting to a hermetic motor compressor or the like and a detection unit 3 serving as a fluid level detecting part. The hermetic terminal 2 includes a cup-shaped metal substrate 4 and electrically conductive pins 6 extending through the metal substrate 4 and fixed in an insulating manner by an electrically insulation filler 5 such as glass.

The detection unit 3 includes two metal fluid level detection electrode plates 7 extending in parallel to each other and two spacers 8 fixed to both ends of the electrode plates 7 respectively. The spacers 8 have respective holes 8A into which distal ends of the electrode plates 7 are inserted to be fixed. Thus, a positional relationship between the electrode plates 7 is determined as the result of constituting the detection unit 3 together with the spacers 8. Each electrode plate 7 is thick to have a strength such that each electrode plate 7 does not bow in a normal use. As a result, the box shape of the detection unit 3 and the positional relationship between the electrode plates 7 can be reliably retained. Connecting terminals 9 are welded and fixed to the electrode plates 7 respectively.

The detection unit 3 is connected via the connecting terminals 9 to the conductive pins 6 of the airtight terminal 2 by welding thereby to be fixed, respectively. The connecting terminals 9 are formed by bending metal plates at bends 9A so as to have generally L-shaped sections, respectively. Since the connecting terminals 9 are welded to the respective conductive pins 6 while in parallel to the electrode plates 7, the welding work can be rendered easier without interference of a welding electrode with the electrode plates 7 and the conductive pins 6. Displacement caused during welding or stress applied to the detection unit in normal handling or normal use would deform the electrodes. However, since the connecting terminals 9 are thinner than the electrode plates 7 and elastically bendable, the connecting terminals 9 are deformed before deformation of the electrodes due to the aforesaid stress. This can reduce deformation of the electrodes and resultant changes in the electrostatic capacitance between the electrodes. Accordingly, even if expansion of the metal hermetic terminal 2 due to an increase in the pressure in a compressor casing in the application to a hermetic motor compressor slightly displaces or changes the positional relationship between the conductive pins 6, a stable reference value can be obtained without substantial influences on the interelectrode distance.

Embodiment 1

FIG. 4 shows a fluid level sensor 21 in which the bends 29A of the connecting terminals 29 are diverted to a large extent so that the lengths of the connecting terminals 29 are rendered longer than an actual distance between secured portions. As a result, since each entire connecting terminal becomes easier to bow against a force applied thereto, stress applied to each connecting terminal 29 is widely dispersed without concentration on a part of each connecting terminal 29.

Embodiment 2

High technological capability is required in order to obtain an adequate retention strength by combining two connecting terminals after the connecting terminals have been constructed to attain balances between strength and elasticity. In view this, one connecting terminal 39A is constructed to have a high stiffness by increasing the plate thickness in a fluid level sensor 31 as shown in FIG. 5. Furthermore, the other connecting terminal 39B is constructed to be thinner and more elastic as well as has an increased length by diverting the bend 39B1 to a large extent in the same manner as the foregoing embodiment. Accordingly, while the connecting terminal 39A can strongly fix the detection unit 3 to the lead terminals 6, the connecting terminal 39B can elastically absorb warp caused during welding and dimensional changes. Although the connecting terminal 39B is formed by machining a metal plate in the embodiment, the connecting terminal 39B may be constructed to use plastic deformation by a more flexible conductive wire or the like.

Furthermore, although the distal ends of the electrode plates are inserted into the insulating spacers to be fixed thereby to constitute the detection unit in the foregoing embodiments, parts to be inserted are not necessarily limited to the distal ends if the detection unit 3 can be handled without change in the positional relationship between the electrodes during the fixing and other handling thereof. For example, the electrode plates and the spacers may be structured so that the distal ends of the electrode plates protrude through the spacers respectively. 

1. An electrostatic capacitance fluid level sensor in which a hermetic terminal includes a metal plate and electrically conductive terminal pins inserted through the metal plate to be hermetically insulated and fixed and electrodes provided on the hermetic terminal for detection of fluid level by electrostatic capacitance, the sensor comprising: a fluid level detection part mounted on the hermetic terminal and constituted into a detection unit including two electrodes electrically insulating spacers fixing a positional relationship between the electrodes; and at least one connecting terminal configured to connect and fixing the detection unit to the two conductive pins, the connecting terminal being deformable by a weaker force than a force causing the electrodes to deform.
 2. The sensor according to claim 1, wherein the connecting terminal is formed of a thinner metal plate than the electrode thereby to be deformable.
 3. The sensor according to claim 1, wherein the connecting terminal includes a bend diverted to a predetermined extent so that the connecting terminal has a length longer than an actual distance between secured portions.
 4. (canceled)
 5. The sensor according to claim 2, wherein the connecting terminal includes a bend diverted to a predetermined extent so that the connecting terminal has a length longer than an actual distance between secured portions.
 6. The sensor according to claim 1, wherein two connecting terminals are provided, one of the connecting terminals has a sufficient stiffness to hold the detection unit and the other connecting terminal is flexible enough to be deformable.
 7. The sensor according to claim 2, wherein two connecting terminals are provided, one of the connecting terminals has a sufficient stiffness to hold the detection unit and the other connecting terminal is flexible enough to be deformable.
 8. The sensor according to claim 3, wherein two connecting terminals are provided, one of the connecting terminals has a sufficient stiffness to hold the detection unit and the other connecting terminal is flexible enough to be deformable.
 9. The sensor according to claim 5, wherein two connecting terminals are provided, one of the connecting terminals has a sufficient stiffness to hold the detection unit and the other connecting terminal is flexible enough to be deformable. 